Method for synthesizing bimetal catalyst particles made of platinum and of another metal and use thereof in an electrochemical hydrogen production method

ABSTRACT

A process for the synthesis of particles of bimetal catalyst based on platinum and on at least one second metal comprises the chemical reduction of a first platinum-based salt or complex and of at least one second salt or complex based on the second metal, the chemical reduction comprising the following stages: the preparation of a mixture comprising the first platinum-based salt or complex and the second salt or complex based on the second metal, in the presence of a pure reducing agent in the liquid form under ambient temperature and pressure conditions, the conditions being respectively defined as equal to 25° C. and 100 kPa; bringing the mixture to a temperature between approximately the freezing temperature of water and the freezing temperature of the reducing agent.

The field of the invention is that of H₂/O₂ fuel cells. The use of this type of fuel cell in the automobile industry instead of and in place of internal combustion engines is still encountering problems related to the storage of the hydrogen: pressurized hydrogen tanks (gaseous storage) are potentially dangerous and metal hydrides (storage in solid form) are inappropriate due to their low energy density, as described in the paper by Schlapbach L. and Züttel A., Hydrogen-storage materials for mobile applications, Nature, 2001, 414, 353-8.

The production of hydrogen onboard the vehicle by catalytic reforming offers an alternative solution to the direct storage of hydrogen, as described in the paper by Basile A., Galluci F. and Paturzo L., “Hydrogen production from methanol by oxidative steam reforming carried out in a membrane reactor”, Catalysis Today, 2005, 104, 251-9, but a purification stage is necessary in order to feed the fuel cell.

For this type of application, gaseous biofuels or hydrocarbons, such as natural gas, or liquid biofuels and hydrocarbons, such as alcohol, gasoline or diesel oil, are potentially sources of hydrogen and an electrochemical cell connected to a low-power electrical supply can be used to extract, at low temperature, hydrogen from the gas mixture comprising carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄) and other gases.

By applying a sufficient electric voltage to the terminals of the cell, the electro-oxidation of hydrogen and of the carbon monoxide take place at the anode:

H₂→2H⁺+2e⁻(E° (H⁺/H₂)=0 vs. SHE, electrochemical equilibrium standard potential, SHE being the potential of the standard hydrogen electrode)

CO_(ads)+H₂O→CO₂+2H⁺+2e⁻

as described in the paper by M. Ciureanu et al., “Electrochemical Impedance Study of Electrode-Membrane Assemblies in PEM Fuel Cells I. Electro-oxidation of H₂ and H₂/CO Mixtures on Pt-Based Gas-Diffusion Electrodes”, Journal of the Electrochemical Society, 1999, 146, 4031-4040.

Platinum is very often used as reaction catalyst in electrochemical systems but its use in a purification application as anode material presents a problem, although it is the best material used at low temperature for the electro-oxidation reaction of hydrogen (H₂→2H⁺+2e⁻).

This is because platinum (Pt) is expensive, rare and less effective at low potential because of the ability which carbon monoxide (present in the reformed hydrogen) has to poison it. This poisoning takes place by irreversible adsorption of the CO at the surface of the Pt, blocking the adsorption sites available, thus preventing the adsorption of the hydrogen and its oxidation.

In point of fact, the electro-oxidation of CO at the surface of Pt supported on carbon, that is to say of CO to CO₂ (CO+H₂O→CO₂+2H⁺+2e⁻) takes place at relatively high potentials located around from 0.7 to 0.8 V vs. SHE, which requires a not insignificant energy contribution.

In order to overcome the difficulties encountered with platinum Pt related essentially to its poor tolerance toward carbon monoxide CO, novel anode catalysts are being looked for.

In order to result in these novel types of catalysts, the preparation of an alloy of Pt with a transition metal can be envisaged or the combination of highly divided Pt with a phase of the metal oxide type. Several platinum alloys, such as platinum-ruthenium (Pt—Ru), platinum-tin (Pt—Sn), platinum-molybdenum (Pt—Mo) or platinum-cobalt (Pt—Co), supported on carbon black with a high specific surface, are forming the subject of studies with the aim of finding the best platinum-based alloy which has good tolerance toward carbon monoxide CO and which makes it possible, moreover, to oxidize said carbon monoxide CO at low potential.

Furthermore, it is known that the performance of these anodic catalysts with respect to an electro-oxidation of H₂/CO depends on their structure, on their chemical composition, on the nanometric size of the particles and on the nature of their support. Command of the technique for elaboration and of the method for the preparation of the catalysts makes it possible to control the physicochemical properties. The metal nanoparticles are generally synthesized by chemical reduction of metal salts or complexes in the presence of a reducing agent. During the synthesis, the mechanism of development of the nanoparticles takes place by germination and growth. This stage is important as it determines the size of the particles and consequently impacts the electroactive surface of the catalyst.

The production of catalyst nanoparticles by chemical reduction of metal salts or complexes in the presence of a reducing agent is a method which is simple to carry out but the command and the control of the phases of germination and growth of the nanoparticles remain a major issue.

For this reason and in this context, the subject matter of the present invention is a method based on the optimization of the operating conditions of the synthesis in order to limit the phase of growth of the nanoparticles and to promote that of the germination, making it possible to increase the number of seeds.

In comparison with a spontaneous development during the synthesis of particles by chemical reduction (without modifying the operating parameters), the limitation of the growth phase makes it possible to obtain nanoparticles with small sizes, typically of the order of 2 to 5 nm, and a greater number of particles.

Uniting these two aspects offers a microstructural configuration in which the electroactive surface of the catalyst is optimal.

Modifying the temperature at which the chemical reduction of the metal salts and complexes takes place represents the most effective means for reducing the rate of growth of the particles as the two parameters develop in the same direction.

More specifically, a subject matter of the present invention is a process for the synthesis of particles of bimetal catalyst based on platinum and on at least one second metal, characterized in that it comprises the chemical reduction of a first platinum-based salt or complex and of at least one second salt or complex based on said second metal, said chemical reduction comprising the following stages:

-   -   the preparation of a mixture comprising said first         platinum-based salt or complex and said second salt or complex         based on said second metal, in the presence of a pure reducing         agent in the liquid form under ambient temperature and pressure         conditions (ATPCs), said conditions being defined at 25° C. and         100 kPa;     -   bringing said mixture to a temperature between approximately the         freezing temperature of water and the freezing temperature of         the reducing agent.

According to an alternative form of the invention, the reducing agent is formic acid, the temperature of said chemical reduction being carried out at a temperature of between approximately 0° C. and 8° C., which can advantageously be of the order of 4° C.

According to an alternative form of the invention, the reducing agent is hydrazine, the temperature of said reduction being carried out at a temperature of between 0° C. and 2° C.

According to an alternative form of the invention, the reducing agent is formaldehyde, the temperature of said reduction being carried out at a temperature of between −19° C. and 0° C.

According to an alternative form of the invention, the process comprises the mixing of platinum salt or complex and of salt or complex of said second metal, in the presence of particles of carbon black or of metal oxide or of metal nitride or of metal carbide.

According to an alternative form of the invention, the amount of reducing agent is greater than or equal to the amount necessary to carry out the chemical reduction of all of the platinum salts or complexes and of the salts or complexes of the second metal.

According to an alternative form of the invention, the reaction is carried out in the presence of an additional energy source which makes it possible to accelerate the chemical reduction operation without promoting the growth of nanoparticles.

According to an alternative form of the invention, the additional energy source is an ultraviolet radiation source.

According to an alternative form of the invention, the ultraviolet radiation source emits in a wavelength range of between approximately 200 nm and 300 nm.

According to alternative form of the invention, the second metal is tin or ruthenium or molybdenum or cobalt.

According to an alternative form of the invention, the particles of bimetal catalyst are based on platinum and tin and their size distribution exhibits a median value of 4 nm with a low dispersion: the standard deviation is of the order of 1.1.

Another subject matter of the invention is the use of the process for the synthesis of particles of bimetal catalyst according to the invention, in a method for the electrochemical production of hydrogen comprising a catalytic reforming reaction in the presence of said catalyst particles and of a gas mixture comprising hydrocarbon compounds. This is because the metal particles obtained by the method of synthesis of the invention appear to be less sensitive to contamination by the hydrocarbon compounds present in the gas mixture than the particles obtained according to the methods described in the state of the art.

According to an alternative form, the gas mixture comprises carbon monoxide, carbon dioxide and methane.

A better understanding of the invention will be obtained and other advantages will become apparent on reading the description which will follow, given without implied limitation, and by virtue of the appended figures, among which:

FIG. 1 illustrates the UV absorption spectra of the PtCl₆ ²⁻ ions;

FIG. 2 illustrates the distribution in a size of particles of the Pt₃Sn/C catalysts synthesized at ambient temperature and at 4° C., according to the process of the invention;

FIG. 3 illustrates the cyclic voltammogram obtained with regard to Pt₃Sn/C synthesized at ambient temperature in an aqueous H₂SO₄ solution with a concentration of 0.5M at 25° C.;

FIG. 4 illustrates the cyclic voltammogram obtained with regard to Pt₃Sn/C synthesized at 4° C. in an aqueous H₂SO₄ solution with a concentration of 0.5M at 25° C.;

FIG. 5 illustrates the change in the current for the electro-oxidation of H₂ over Pt₃Sn/C synthesized at ambient temperature and at 4° C. in aqueous H₂SO₄ solution with a concentration of 0.5M after having subjected the catalyst to a gas mixture composed of 50 ppm of CO in H₂ and at 0.24 V vs. RHE and reached a virtually stationary state of the change in the hydrogen oxidation current over time.

For the synthesis of a platinum-metal (Pt—M) alloy catalyst, the Applicant uses the FAM (Formic Acid Method) method as described in the paper by E. I. Santiago et al., “CO tolerance on PtMo/C electrocatalysts prepared by the formic acid method”, Electrochimica Acta, 48 (2003), 3527-3534.

According to the present invention, it is proposed to use solutions of salts or complexes of Pt and of a metal ally as precursors of Pt—M catalysts supported or not on carbon black with a high specific surface, a metal oxide (TiO₂, ZrO₂, Al₂O₃, and the like), metal nitrides (TiN, TaN, BN) or metal carbides (TiC, WC, W₂C, Mo₂C, and the like).

In the case of a carbon support, the solutions of salts or complexes of Pt and of the allied metal element M are mixed with the carbon support and the combination is vigorously stirred with ultrasound for at least half an hour, the time necessary in order to obtain a homogeneous mixture.

The volume of the solutions is determined from the concentration of the solutions and so as to obtain the desired atomic composition of the metal alloy. The weight of support of metal nanoparticles of use in the synthesis is determined so that it represents 50% of the weight of the catalyst synthesized. Formic acid is used as reducing agent and is added to the mixture described above.

The volume of formic acid must be in excess in order for the chemical reduction of the metal salts or complexes to be complete.

The whole of the mixture is subsequently brought to a temperature between the freezing temperature of water and that of formic acid. After from 12 to 72 hours, the chemical reduction is complete and a metal powder representing the catalyst is obtained.

Lowering the temperature promotes the decrease in the rate of growth of the nanoparticles and consequently lengthens the duration of chemical reduction of the metal salts. In order to overcome this lengthening of duration, it is advisable to accelerate the chemical reduction reaction without, however, increasing the rate of growth.

The flask containing the mixture can advantageously be exposed to an energy source, for example a source of ultraviolet (UV) rays, which makes it possible, by virtue of this energy contribution, to accelerate the reaction (the germination of the particles), thus promoting the multiplicity of seeds in the nanoparticulate state without, however, promoting their growth. The wavelength of the UV rays is preferably chosen between 200 and 300 nm, corresponding to the absorption region of the platinum Pt complex, as shown in FIG. 1.

Implementational Example

Pt—Sn/C catalysts with the molar composition 3:1 were synthesized by chemical reduction with formic acid.

Solutions of K₂PtCl₆.6H₂O and SnCl₂.2H₂O from Sigma-Aldrich were used as precursors of the catalysts formed of Pt—Sn supported on carbon black with a high specific surface (Vulcan XC-72R, Cabot Corp., 250 m²/g).

The aqueous solutions of platinum and tin salts with a concentration of 0.01 M were mixed in the presence of carbon black and stirred vigorously under ultrasound for a period of time of approximately 1 hour. The volumes of the K₂PtCl₆.6H₂O and SnCl₂.2H₂O solutions mixed are respectively 15 ml and 5 ml, so as to obtain an atomic ratio of 3:1.

A large amount of formic acid HCOOH (ACS reagent, greater than or equal to 98%, Sigma-Aldrich), with a molar ratio of the order of 1000:1 between the formic acid and the metal salts, used as reducing agent, is added to the mixture in order to make possible simultaneous reduction of the platinum and tin salts.

Two examples of synthesis operating conditions were carried out:

-   -   the first example at ambient temperature for 24 hours;     -   the second example at 4° C. for 72 hours. At the end of the         period of time specified above, a metal powder was obtained. The         weight of Pt+Sn represents 50% by weight of the catalyst. The         temperature of 4° C. was chosen so that it is between 0 and 8°         C.

The objective of carrying out the synthesis at this temperature is to reduce, during the synthesis, the rate of growth of the nanoparticles, which has a growth with temperature dependence.

The results obtained are listed in the table below and relate to the physicochemical properties of the Pt₃Sn/C catalysts synthesized at ambient temperature and at 4° C.

Electroactive Mean size of surface Electro-oxidation the particles (Hupd) potential of CO in Catalyst (nm) (cm² _(Pt). cm⁻² _(geo)) V (volts) Pt₃Sn/C 5.9 266 0.38 (Ambient temperature) Pt₃Sn/C at 4° C. 3.9 379 0.28

The electroactive surface corresponds more specifically to the surface which is electrochemically active for the reactions under consideration, which it is desired to increase.

FIG. 2 illustrates the size distributions of particles of Pt₃Sn/C catalysts synthesized at ambient temperature (25° C.) and at 4° C. and demonstrates the high percentage of particles of small size, typically from 3 to 4 nm, with the synthesis process of the invention.

More specifically, the size parameters listed in the table below are obtained:

Pt₃Sn/C (25° C.) Pt₃Sn/C (4° C.) Mean size 5.89 3.92 Median size 6.00 4.00 Standard deviation 1.45 1.10

Measurements carried out by energy dispersive X-ray spectrometry (EDS) analysis also provide the results below:

EDS composition Unit cell parameter Catalyst (Pt:Sn at. %) (A) Pt₃Sn/C (25° C.) 85:15 3.99231 Pt₃Sn/C (4° C.) 74:26 3.99231

The electroactive surface of the catalyst prepared at 4° C. is thus much greater than that of the same catalyst synthesized at 25° C.

This means that the amount of catalyst necessary, for example for the satisfactory operation of a system for the electrochemical production of hydrogen comprising a catalytic reforming reaction in the presence of particles of catalyst obtained according to the present invention and of a gas mixture comprising hydrocarbon compounds, can thus advantageously be lower than that used with a catalyst obtained with a process of the prior art.

The Applicant has produced electrochemical half-cell cyclic voltammograms at 25° C. with a scan rate of 10 mV/s, relating to the different catalysts prepared.

It should be remembered that cyclic voltammetry is an electrochemical analysis method based on the measurement of the current resulting from the reduction or the oxidation of the compounds which come into contact with a working electrode (the sample studied) under the effect of a controlled variation in the difference in potential with an electrode with the set potential, known as reference electrode. It makes it possible to identify and to quantitatively measure a large number of compounds and also to study the chemical reactions including these compounds.

The high absorption power can be characterized by the absence of oxidation peaks (positive current) on the voltammogram exhibiting the current density measured as a function of the potential applied E at the working electrode. The absence of these peaks on the voltammogram reflects the blocking of the adsorption sites of the catalyst by another entity.

FIG. 3 relates to the results obtained with the Pt₃Sn/C particles prepared at 25° C. The curve 3 a relates to the voltammetry curve (under N₂, in 0.5M H₂SO₄ at 25° C. and 10 mV.s⁻¹) produced after contamination of the catalyst by adsorption of CO.

The comparison may be established with the curve 3 b produced after desorption of the CO (voltammetry cycle: VC).

The catalyst is completely contaminated: no oxidation current is observed on the curve 3 a in the potential range corresponding to the desorption of electro-adsorbed hydrogen (Hupd: underpotentially deposited H): between 0.1 and 0.4 V vs. RHE. The oxidation of the CO begins when the potential of the electrode exceeds 0.4 V vs. RHE.

FIG. 4 relates to the results obtained with the Pt₃Sn/C particles prepared at 4° C. For the catalyst prepared at 4° C., the same cyclic voltammetry curve after contamination of the catalyst by the adsorption of CO (curve 4 a) shows that the catalyst is not totally contaminated: the peaks for desorption of the hydrogen are still observed on the curve 4 a between 0.1 and 0.4 V vs. RHE and the oxidation of the CO begins at a potential of the electrode of 0.3 V vs. RHE.

The comparison can be established with the curve 4 b produced after desorption of the CO (voltammetry cycle: VC).

The gain, in energy terms, for the oxidation of the hydrogen in the presence of traces of contaminating gases is close to 30% (0.72 kWh/Sm³ _(H2) with the catalyst synthesized at 4° C., versus 0.96 kWh//Sm³ _(H2) with the catalyst synthesized at 25° C.).

The Applicant has also monitored the change in the current for the electro-oxidation of H₂ over Pt₃Sn/C synthesized at ambient temperature and at 4° C. in an electrochemical half-cell device, when the electrode is supplied with a gas mixture composed of 50 ppm of CO in H₂ and subjected to a potential of 0.24 V vs. RHE.

FIG. 5 shows, via the curves 5 a and 5 b, the difference in behavior with a catalyst produced according to the present invention at a temperature of 4° C., in comparison with a catalyst produced at ambient temperature.

The current density measured over time (j) is relative to that measured when the sample is supplied with pure hydrogen (j_(max)). Initially, the catalyst is not contaminated; thus j=j_(max) and j_(jmax)=1.

In the case of the catalyst synthesized at 25° C. (curve 5 a), the current density measured over time rapidly decreases to reach 71% of the initial current after 1 hour of poisoning with CO. For the catalyst synthesized at 4° C., the current measured after 1 hour of poisoning is 93% of the initial current. The catalyst synthesized at 4° C. is thus much more tolerant toward CO than that synthesized at 25° C. (FIG. 5 b). 

1. A process for the synthesis of particles of bimetal catalyst based on platinum and on at least one second metal, comprising chemical reduction of a first platinum-based salt or complex and of at least one second salt or complex based on said second metal, said chemical reduction comprising the following stages: preparing a mixture comprising said first platinum-based salt or complex and said second salt or complex based on said second metal, in the presence of a pure reducing agent in the liquid form under ambient temperature and pressure conditions, said conditions being respectively defined as equal to 25° C. and 100 kPa; and bringing said mixture to a temperature between approximately the freezing temperature of water and the freezing temperature of the reducing agent.
 2. The process for the synthesis of particles of bimetal catalyst as claimed in claim 1, wherein the reducing agent is formic acid, the bringing to a temperature being carried out between approximately 0° C. and 8° C.
 3. The process for the synthesis of particles of bimetal catalyst as claimed in claim 1, wherein the reducing agent is hydrazine (N₂H₄), the bringing to a temperature being carried out between approximately 0° C. and 2° C.
 4. The process for the synthesis of particles of bimetal catalyst as claimed in claim 1, further comprising the mixing of platinum salt or complex and of salt or complex of said second metal, in the presence of particles of carbon black or of metal oxide or of metal nitride or of metal carbide.
 5. The process for the synthesis of particles of bimetal catalyst as claimed in claim 1, wherein the amount of reducing agent is greater than or equal to the amount necessary to carry out the chemical reduction of all of the platinum salts or complexes and of the salts or complexes of the second metal.
 6. The process for the synthesis of particles of bimetal catalyst as claimed in claim 1, wherein the reaction is carried out in the presence of an additional energy source which makes it possible to accelerate the chemical reduction operation without promoting the growth of nanoparticles.
 7. The process for the synthesis of particles of bimetal catalyst as claimed in claim 6, wherein the additional energy source is an ultraviolet radiation source.
 8. The process for the synthesis of particles of bimetal catalyst as claimed in claim 7, wherein the ultraviolet radiation source emits in a wavelength range of between approximately 200 nm and 300 nm.
 9. The process for the synthesis of particles of bimetal catalyst as claimed in claim 1, wherein the second metal is tin or ruthenium or molybdenum or cobalt.
 10. The process for the synthesis of particles of bimetal catalyst as claimed in claim 9, wherein the size distribution of said particles exhibits a median size of 4 nm and a standard deviation of 1.1.
 11. The use of the process for the synthesis of particles of bimetal catalyst as claimed in claim 1, in a method for the electrochemical production of hydrogen comprising a catalytic reforming reaction in the presence of said catalyst particles and of a gas mixture comprising hydrocarbon compounds.
 12. The use of the process for the synthesis of particles of bimetal catalyst as claimed in claim 11, wherein the gas mixture comprises carbon monoxide, carbon dioxide and methane. 